Paul ends the letter by giving glory to God for the Gospel, which he describes in a startling way for anyone interested in the theology of Scripture and its interpretation.
Now to him who is able to establish you by my gospel and the proclamation of Jesus Christ, according to the revelation of the mystery hidden for long ages past, but now revealed and made known through the prophetic writings by the command of the eternal God, so that all nations might believe and obey him - to the only wise God be glory forever through Jesus Christ! Amen. (Rom 16:25-27)
I understand Paul to be saying that the good news (proclamation) of Jesus Christ, God's provision of a vicarious righteousness in fulfilment of his promises given to Israel throughout its existence, is now revealed in the Old Testament Scriptures as a mystery that had been hidden for a long time before. It is striking to think of this mystery being revealed in Paul's time through Scriptures that had been written hundreds of years earlier. It is not that the revelation had been given earlier and was now illumined by the coming of Christ. Paul says in his introduction that the Gospel had been promised before hand through the OT(1:2), but in light of the letter's conclusion I have to think that the promise was given as mystery hidden until its fulfilment. Though the texts had been written long before, they only revealed that mystery at the resurrection of Christ (1:4). The coming of Christ truly opened the Scriptures to his followers and gave it meaning it could not have previously had. I take Paul here to be prohibiting us from imagining that if those Jews living prior to the resurrection of Christ had just read the relevant messianic texts from the Old Testament (passages from Deuteronomy, the Psalms, Isaiah, Jeremiah, Daniel, and a few smatterings from the Minor Prophets) more clearly, they would have understood that God was going to send his Son to provide a righteousness through faith in Christ's vicarious death and resurrection before it happened. No, this was impossible to conceive, and thus impossible for the Old Testament to reveal, until it happened. But when it did happen, these texts written long ago in ages past revealed what had now just recently happened. Staggering.